At present, because of their great hardness and ability to withstand high temperatures, ceramics are being used increasingly for making cutting tools. As a general rule, known ceramic cutting tools that enable high speed machining to be performed on very hard materials, such as those described in document EP-B1-0 477 093, are milling cutters and lathe tools. Unfortunately, the stresses that can be applied to a drill bit (drilling depth, swarf removal, intensity and direction of cutting forces) during a drilling operation are greater than those that might be applied, for example, to a lathe tool or to a cutter during a milling operation. These stresses make it more difficult to provide ceramic bits for drilling at very high speeds in very hard materials.
Numerous manufacturers propose ceramic drill bits in their catalogs, and document U.S. Pat. No. 5,641,251 describes one such bit. Although those bits provide improved performance compared with conventional bits made of high-speed steel or of tungsten carbide, they are limited in their utilizations and they do not enable very high speed drilling to be performed in materials as hard as superalloys. Because of the low toughness of ceramic materials, ceramic bits present lower torsional strength and compressive strength than metallic bits, e.g. made of tungsten carbide, with these mechanical characteristics causing ceramic bits to be brittle when drilling in hard materials or when drilling at high speeds of advance or at high cutting speeds. Work has been undertaken to improve the mechanical characteristics of ceramic-based materials: document U.S. Pat. No. 4,789,277 describes ceramics in which fibers (or whiskers) of silicon carbide (SiC) are introduced to improve their mechanical characteristics. In addition, it is known and recommended to ensure that the cutting edges of drill bits are always made with zero or negative angles so as to protect the cutting edges from wear, thereby increasing the lifetime of a ceramic bit.
Nevertheless, such bits are still of limited use in terms of the materials they can drill and of the cutting and advance speeds of the bits. When drilling materials as hard as refractory materials such as superalloys based, for example, on nickel and cobalt (having a Vickers hardness number of about 440) and when the cutting and advance speeds are very high, for example when the cutting speed is greater than about 400 meters per minute (m/min) and when the speed of advance is greater than 0.04 millimeters per revolution, the twisting and axial compression forces that are generated and applied to prior art bits are such that they will inevitably break. In addition, the cutting forces exerted by such bits on the workpieces to be drilled, and the friction between the radially-outer surfaces of the bits and the inner cylindrical surfaces of the drilled holes lead to thermal stresses in the bits and the workpieces to be drilled that cause accelerated degradation of the bits and deformation of the workpiece when attempts are made to drill at high speed in very hard materials.
In addition, with increasing depth of a hole being drilled, the twisting forces applied to ceramic bits become ever greater, firstly because there is an ever increasing outside area of the bit rubbing against the inner cylindrical surface of the drill hole, and secondly because when drilling at high speeds, prior art ceramic bits cannot evacuate a large quantity of swarf efficiently, thereby leading to clogging phenomena in the bit, and consequently increasing the twisting forces applied to the bit and increasing the risks of it breaking. These drawbacks generally make it impossible to drill holes at high speed to a depth that is greater than the diameter of the bit.